Fix select fields regression on master backlog/taskboard after merging #942.

Added check to ensure default select value does not override values of existing stories/tasks.
Change travis to use RM 2.3.2

Redmine has pinned nokogiri at '< 1.6.0'. Backlogs does the same for now.
added tests for the default story tracker
Merge remote-tracking branch 'stefan-langenmaier/27130-invert-story-bug'
Merge remote-tracking branch 'origin/master'